A variety of mount assemblies are presently available to isolate vehicle vibrations, such as for automobile and truck engines and transmissions. One of the most popular mounts today is the hydraulic-elastomeric mount of the type disclosed in U.S. Pat. No. 4,588,173 to Gold et al., issued May 13, 1986 and entitled "Hydraulic-Elastomeric Mount".
The hydraulic mount assembly of this prior invention includes a reinforced, hollow rubber body that is closed by a resilient diaphragm so as to form a cavity. This cavity is partitioned by a plate into two chambers that are in fluid communication through a relatively large central orifice in the plate. The first or primary chamber is formed between the partition plate and the body. The secondary chamber is formed between the plate and the diaphragm.
A decoupler is positioned in the central orifice of the plate and reciprocates in response to the vibrations. The decoupler movement alone accomodates small volume changes in the two chambers. When, for example, the decoupler moves toward the diaphragm, the volume of the primary chamber increases and the volume of the secondary chamber decreases. In this way, at certain small vibratory amplitudes and high frequencies, fluid flow between the chambers is substantially avoided and undesirable hydraulic damping is eliminated. In effect, this freely floating decoupler is a passive tuning device.
In addition to the large central orifice, an orifice track with a smaller flow passage is provided, extending around the perimeter of the orifice plate. Each end of the track has one opening; one communicating with the primary chamber and the other with the secondary chamber. The orifice track provides the hydraulic mount assembly with another passive tuning component, and when combined with the freely floating decoupler provides at least three distinct dynamic modes of operation. The operating mode is primarily determined by the flow of the fluid between the two chambers.
More specifically, small amplitude vibrating inputs, such as from smooth engine idling or the like, produce no damping due to decoupling. On the other hand, large amplitude vibrating inputs produce high volume, high velocity fluid flow through the orifice track, and accordingly a high level of damping force and smoothing action. The high inertia of the hydraulic fluid passing through the orifice track contributes to the relatively hard mount characteristic in this mode. As a third (intermediate) operational mode of the mount, medium amplitude inputs produce lower velocity fluid flow through the orifice track generally resulting in a medium level of damping. In each instance, as the decoupler moves from one seated position to the other, a relatively limited amount of fluid can bypass the orifice track by moving around the sides of the decoupler to smooth the transition between the operational modes.
Recent developments in hydraulic mount technology have led to the advent of electronic control of the damping characteristics of the mount. Such a hydraulic mount is disclosed in the U.S. Pat. No. 4,789,143 issued Dec. 6, 1988, assigned to the assignee of the present invention. This prior invention represents an improvement over previous mounts in that it provides variable damping levels in response to sensed vehicle operating conditions. This active tuning of the mount is clearly a more sophisticated approach and has found general acceptance among engineers and others as an advance in the art. The tuning is actually accomplished by the use of an infinitely variable sliding gate for selectively varying the size of the opening to the orifice track between the two chambers. By varying the opening size, the flow of damping fluid and thus the damping action of the mount can be changed.
Another approach to active tuning involves providing an inflatable bellows in the primary chamber of the mount; Hydraulic Engine Mount with Air Bellows Tuning, Smith et al, Ser. No. 240,668, filed Sept. 6, 1988. Transducers and an electronic controller regulate the flow of air into/out of the bellows in order to control the damping effect of the mount.
Not only are these prior art mounts with active control proven to be successful in further modulating the response of the mount to vehicle operating conditions, but they can be programmed to operate in a manner particularly adapted to the vehicle configuration and the particular component, such as a motor or transmission, being damped. However, the disadvantage of these new and more sophisticated systems is the relatively higher cost of manufacturing and maintenance.
Thus, it would be desirable to have an alternative approach to these active systems. The alternative mount would employ passive damping features so as to be less expensive, but would be characterized by the ability to be designed and engineered to fit a particular set of operating parameters and conditions of a vehicle or component. Advantageously, the passive tuning means is to be self-contained and operate efficiently, without resort to electronic controllers, external transducers, microprocessors or the like.